Measuring of surfaces may be performed by using a laser light section method. This may give rise to problems. For example, with regard to intensities and reflectances, a high intensity of the laser light line may result in outshining and/or saturation to a maximum value in the image sensor used. This may result in that evaluation algorithms such as center of gravity (CoG) determination no longer work correctly. If a fixed threshold value is used for determining the position, a large width of the laser light line comprising a constant brightness value may be ascertained, so that a position of the laser light line may be determined with difficulty only or in an imprecise manner. Low intensity of the laser light line may cause only a very small level to be sensed, so that a pronounced influence of the noise will prevail. This leads to uncertainty in the determination of the center of gravity due to the noise. In addition, large dynamics within the image, e.g. of 200:1 and more, may result in that differences in the image can no longer be balanced off and/or that regions having small and large intensities will arise, which may lead to the difficulties described above.
If one looks at the evaluation of a transfer function, which may be used as a possibility of correcting the problem of contrast, this will be possible in various modes of operation. There is the possibility of linear operation. This offers the advantage that by means of CoG, there is an efficient method at hand for accurately determining the location of a Gaussian laser line. However, this entails the disadvantage that in the event of a superposition of a reflectance leap and a laser line, there will be a change in the signal value, as may be shown by the following formula:
      S    =          RE      K        ,wherein S designates the signal value, R designates the reflectance of the surface, E designates irradiated energy (Ee), and K designates a normalization quantity. Said change, or step, in the signal value results in a shift in the optical center of gravity and, therefore, in an error in determining the position of the laser line. Outshining in the bright areas also causes CoG determination to be erroneous or to no longer work. Underexposure of the dark areas results in that no more signal will be visible. As of a specific value, the contrast prevents that all of the brightness stages can be captured in one image.
There is further the possibility of logarithmic operation, which offers the advantage that a solution to the problem of contrast may be obtained. In the formula
            S      l        =          log      ⁡              (                  RE          K                )              ,Sl describes a logarithmic signal value. Said evaluation enables compression of the signal, so that a larger contrast within the image no longer results in suppression of the dark areas. This entails the disadvantage that CoG can no longer be applied due to the non-linear characteristic on the image, so that threshold methods will be applied. However, it is difficult to select suitable thresholds for the threshold methods due to the signal variation caused by the reflectance. Moreover, there is the risk of transient oscillation of the logarithmic (log) circuit.
Moreover, there is the possibility of multilinear operation. This offers the advantage that a solution to the problem of contrast may be obtained by compressing the signal, so that a larger contrast in the image no longer results in suppression of the dark areas. What is disadvantageous, however, is that CoG can no longer be applied due to the non-linear characteristic on the image, so that during logarithmic operation, too, threshold methods have to be applied, which results in difficulty in selecting suitable thresholds due to the signal variation caused by reflection. At the so called breakpoints, i.e. points comprising reflectance leaps, difficulties regarding association of the signals, and, therefore, inaccuracies will arise.
Further, there is the possibility of multi-exposure operation, which offers the advantage that a solution to the problem of contrast is found also by compressing the signal, so that a large contrast in the image no longer results in suppression of the dark areas. However, what is disadvantageous is that due to the non-linear characteristic on the image, CoG can no longer be applied, so that threshold methods are used which result in difficulty in selecting suitable thresholds due to the signal variation caused by reflectance. Also, at the breakpoints there will be difficulties regarding associating of the signals and, thus, there will be inaccuracies. In addition, temporal artifacts will occur due to illumination being effected at different points in time.
Known concepts utilize reading out of brightness information. For describing the process of determining a position x0 of the center of the reproduction of a laser line in column y0 on a sensor matrix, reference will be made below to FIGS. 9a, 9b and 10a to 10c. Known concepts will be explained with reference to FIGS. 11a to 11c. FIG. 9a shows a sensor matrix 100 wherein pixels are arranged in columns along a direction x, several columns being arranged next to one another along a y direction. Thus, pixel and/or column positions are arranged along the x direction, whereas row positions are arranged along the y direction. The pixels have a laser line 102 depicted thereon. It is the aim of the evaluation, for example, to determine, within a column y0, a position x0 of the laser line 102 within the sensor matrix 100.
FIG. 9b shows a schematic diagram of the intensity distribution E within column y0 across the pixel position x. An intensity minimum is depicted, by way of example, by the value 0, and an intensity maximum is depicted by the value 1. The intensity E may also be described as an amplitude of the laser light line received.
In other words, FIGS. 9a and 9b show representations of determining the position x0 of the laser line 102 within column y0 on the sensor matrix 100 of an image sensor.
By way of example, FIGS. 10a to 10c show various possibilities of evaluating the intensity distribution in accordance with FIG. 9b. For example, FIG. 10a shows a linear evaluation of the intensity distribution of FIG. 9b. The progress of the curve essentially corresponds to that shown in FIG. 9b. In addition, the diagram of FIG. 10a shows a progress of a reflectance R at the measured object across the pixel position x.
FIG. 10b shows a schematic representation of pixel values, i.e. signal values S, which may be detected at the image sensor, across the pixel position x. Within the area of the reflectance leap of FIG. 10a, intensity attenuation takes place which results in a shift 106 of the center of gravity, a so called CoG shift.
FIG. 10c shows a logarithmic representation of the signal value Sl. Even in the logarithmic representation, there is a shift 106′, so that the CoG method is not applicable. Due to the logarithmic representation, the attenuated signal A may be enhanced above a noise threshold 108, so that a signal becomes detectable here as well.
FIGS. 11a, 11b and 11c show various possibilities of evaluation. FIG. 11a shows threshold evaluation with regard to a threshold E1′. For reading out the brightness information, one or two threshold values E1′ may be evaluated for determining a position of a rising (xa) and/or falling (xb) edge so as to enable calculation of the mean value x0 of the position of the laser light line. This may be effected with a subpixel accuracy of 0.5 subpixels, as is indicated in FIG. 11b. This can be calculated in a simple and fast manner.
FIG. 11b shows an application of a linear transfer function, i.e. a Gaussian curve, for the CoG algorithm as is depicted, e.g., in FIG. 9b or 10a, so as to determine the maximum. A subpixel accuracy of up to 1/64 may be obtained; in most cases, a subpixel accuracy up to only ⅛ or to 1/16 is reasonable. However, this involves significantly more expenditure than utilizing the edges xa and/or xb and will involve, among other things, analog-to-digital conversion and reading out of the recorded full frame.
FIG. 11c shows utilization of a non-linear transfer function. Implementation may be effected by means of a field-programmable gate array (FPGA) or PC. In the event of a non-linear characteristic, a curve-fitting algorithm may be used, wherein an accuracy of up to 1/64 subpixels may be obtained; here, too, only a range from ⅛ to 1/16 is useful in most cases. Said variant involves more expenditure than utilizing a linear transfer function, also involves analog-to-digital conversion and reading out of the full frame, and may be implemented by means of FPGA or PC. The algorithms described are either imprecise or they involve an analog read-out of the sensor matrix. The algorithms for the linear transfer function and/or the curve-fitting algorithm involve a relatively large amount of expenditure and, in the event of large requirements concerning speed, are implemented in an FPGA in most cases.
In other words, FIGS. 11a to 11c show representations of the variants for determining the position x0 of a laser line along a column.
The fundamental problem in column-by-column search for the exact position of the laser line consists in the nature and reflectance of the surface. Said parameters influence the amount of laser light impinging on the image sensor of the measuring camera from the surface to be measured. The better the reflection properties regarding the solid angle observed by the measuring camera, the more light one will be able to observe. This is depicted in FIG. 12, where at the point marked by A, large reflectance results in large brightness, and where at a point marked by B, little reflectance results in little brightness. For determining the position of the laser line, one uses the brightness distribution along the line 112 between points A and B, which is depicted at the abscissa in the lower area of FIG. 12. In other words, FIG. 12 shows laser lines on surfaces having different reflectances in the upper area and shows a section through both lines in the lower area. The reflectance at point A is comparatively large, and that at point B is comparatively small.
FIGS. 10a to 10c depict the fundamental problems. In the event of a leap of the reflectance of the surface examined, the signal of the laser line will also vary, so that in the event of evaluation by means of CoG or the threshold method, the position of the maximum will shift. In other words, FIGS. 10a to 10c show a progress of the brightness in the event of a reflectance leap, wherein FIG. 10a shows the amplitude of the laser line across the pixel position x and FIG. 10b shows a value within a pixel which is scaled in a linear manner, and FIG. 10c presents said value in a logarithmic ratio.
What would therefore be desirable are devices and methods enabling highly accurate determining of a position of an illumination signal on a surface region.